Introduction

Population growth and rapid urbanization lead to a marked increase in the number of municipal wastewater treatment plants (MWTPs). This increase is accompanied with the generation of a large amount of excess (waste) activated sludge (WAS) that requires proper treatment and management due mainly to its large volume and easiness to decay under normal environmental condition. In industrialized countries, incineration is the main trend of treatment for WAS. In most developing countries where more MWTPs are under construction, however, landfilling and direct damping that cause many environmental and health problems are still widely adopted (Chen et al., 2012).

For stabilization and effectively utilization of WAS that contains rich content of organic matter, aerobic digestion, anaerobic digestion and composting are considered as sustainable disposal options and their applications are increasing. By 2008, 45 % and 21 % of domestic sewage facilities in USA had been using aerobic and anaerobic digestion processes respectively to treat WAS (United Nations Human Settlements Programme, 2008). For all these methods, certain percentages of the organic substances that constitute for the microorganisms, including the major constituting species of cells (proteins, fats and carbohydrates) and extracellular polymeric substances (EPS) (Jin et al., 2016; Jiang, 2007; Laspidou and Rittmann, 2002), are eliminated through involved biological reactions, and some of the intermediates and final products are remaining as dissolved organic matter (DOM) in the leachates or liquids of the WAS treatment systems together with those of less or non-biodegradable organic constituents.

The resulting leachates or liquids are also increasingly concerned when aerobic digestion, anaerobic digestion and composting are adopted for treatment of WAS. If not properly handled, the organic matter contained therein can worsen the water quality of natural water systems and, at the same time, can become the major reason for odors and

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smells (Cheng et al., 2009). Moreover, for drinking water production using source water merged with the leachates or liquids from the biological WAS treatment systems, the DOM also contributes to the enhanced presence of hazardous disinfection byproducts in the water after treatment.

For biological treatment of WAS, so far, many studies have been conducted and the conditions for achieving higher treatment efficiency have been identified (Mohan et al., 2016; Ozdemir et al., 2014; Xiao et al., 2013). However, in regard of the characteristics of the resulting leachates and liquids, systematical studies are very limited; and findings relating to the dependency of the characteristics of the formed DOM on the total solid (TS) concentrations and temperatures of aerobic and anaerobic digestion can be hardly found in available literatures. All these are very important when suitable treatment approaches are considered for lowering the presence level of DOM, thus alleviating the adversary effects of the discharged leachates and liquids to the total environment and to the source water quality of drinking water supplies.

Anaerobic biological treatment of solid organic waste generally involves two important processes: liquefaction (hydrolysis/acidification) and degradation. Hydrolysis is the first step that determines the overall performance of involved bioreactors. In a previous study (Du and Li, 2016a), the authors compared the performance of MFC fed with potato cubes having three different sizes and found that the smaller the size was, the better the performance regarding both electricity generation efficiency and the rate of organic matter removal. Potato cubes with a larger size required a longer time for hydrolysis to occur, leading to obvious decreases of current density and columbic efficiency although the final removal for the fed solid potato was not affected.

For enhancing the performance of MFC with the feedstock of solid potato, methods that can promote the hydrolysis rate of the solid substrate and, at the same time, enable the increased hydrolysis products to be preferentially utilized by electrogenic bacteria rather than by other coexisting heterotrophic bacteria are desired. Thermochemical treatment is a well reported method for promotion of the hydrolysis of various biomass types (including raw food waste, vegetables and waste activated sludge) (Li et al.,

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2016a; Wu et al., 2016). Its effect has also been confirmed by the authors in a recent study of MFC supplied with mixed feedstocks of raw and cooked solid potato (Du and Li, 2016b). In addition to this method, alkali pretreatment and oxidation are also found effective in accelerating hydrolysis, hence improving the efficiency of anaerobic fermentation (Xiao et al., 2013; Li et al., 2016b). However, all these methods have the drawbacks that not only elevate the total operation cost but also make the whole treatment system complicated.

Compared to these methods, mixed feeding of solid potato waste with waste activated sludge (WAS) is probably more effective and applicable. This idea is newly formed based on the following considerations: 1) WAS from municipal wastewater treatment plants (WWTPs) generally contains rich microflora, and some species after simple anaerobic culturing may join the bacteria originally populated in the anode chamber of MFC to promote the hydrolysis of solid potato waste; 2) the amount of WAS generated worldwide is continuously increasing and many developing countries are striving hard to find more effective disposal approaches that can replace conventional practices (including incineration that has been widely adopted in most industrialized countries, and landfilling and direct dumping that have been well used in most developing countries) to realize simultaneous stabilization and resource recovery; and 3) WAS itself contains high content of organic matter and can be used as an unceasing renewable energy source, and sustainable approaches more beneficial to the total environment than conventional ones (e.g. anaerobic digestion, carbonization and composting) are widely expected. For instance, in USA, approximately 7 million tons (dry weight) of sewage sludge is generated per year (Water Environment Research Foundation, 2008), which has a total energy potential of almost 105 MJ (Shen et al., 2015). In China, the total yearly generation of sewage sludge is much larger and had reached more than 30 million tons in 2014 (Cai et al., 2016).

When WAS is mixed into solid potato waste for treatment by MFC, the following points have to be clarified: 1) if WAS can effectively promote the hydrolysis of solid potato and hence elevate the overall performance of MFC; 2) if WAS is used only as an

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additional substrate and degraded together with potato; 3) if the larger number of heterotrophic bacteria in WAS compete with electrogenic bacterial species for hydrolysis products and hence depress electricity generation; and 4) how the composition of the mixed feedstock remaining at the end of the treatment differs from its original. All these points relate to the optimization of the operation conditions expected when MFC is to be adopted for practical applications. However, relevant studies can hardly be found in existing literature although many basic studies relating to the theory and configuration of MFC have been conducted.

Accordingly, in order to generate more useful information relating to the quality of leachates and liquids from biological treatment of WAS, in this study, a comprehensive investigation and comparison of the characteristics of DOM formed and remaining during aerobic and anaerobic digestion of EAS from a representative MWTP was conducted. For this, five aerobic and five anaerobic digestion reactors were operated in parallel under three different TS concentrations (1.2, 2.3 and 5.2 %) and three temperatures (5, 20 and 35 qC). The use of these three temperatures was made in order to cover a broader temperature range for field applications of the biological EAS treatment processes because temperatures not only affect the rate of all involved biological reactions but may also affect the occurrence and fate of intermediates and hence the total composition of DOM. For evaluation of the characteristics of DOM formed and remaining under the different treatment conditions of WAS, in addition to the overall quality indexes of chemical oxygen demand (COD), total organic carbon (TOC), ultraviolet absorbance at the wavelength of 260 nm (UV260), the composition related items, including the specific ultraviolet absorbance (SUVA), volatile fatty acids (VFAs), fluorescence excitation emission matrix (EEM) and molecular weight (MW) distribution, were also analyzed and compared. Moreover, the destruction and lysis of EAS during both aerobic and anaerobic digestion processes were observed using scanning electron microscope (SEM). Finally, principal component analysis (PCA) with the measured values for all water quality indexes of each digestion condition was also

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performed for better understanding their interrelations during digestion.

Comprehensively, the major objective of this study was to investigate the effect of mixed feeding of WAS on the performance of MFC in treatment of solid potato waste.

For this, four two-chamber MFCs fed with mixtures of solid potato with anaerobically cultured WAS from a municipal WWTP with and without undergoing inactivation by chlorine was operated. As control, three MFCs fed respectively with solid potato, the anaerobically cultured WAS and the WAS after inactivation were also operated. The effect on electricity generation was evaluated on the basis of current density, power density and columbic efficiency. And, the treatment efficiency for the solid potato and WAS was evaluated on the basis of COD and volatile fatty acids (VFAs). By analyzing the observed time profiles of COD in the water phase of the anode chamber with a sequential first-order hydrolysis and degradation model, the magnitudes of the rate parameters under different feedstock conditions were estimated. Moreover, with the aid of stable isotope ratio analysis of carbon (Ɂ¹³C), the final composition of the mixed feedstocks remaining after MFC treatment was quantitatively evaluated.